Altimeter Corrections

[Bob - S50]

Just curios...

Short version: Just wondering how many out there have an Altimeter Correction chart in their POH and make adjustments for it when they fly.

Long version: I've noticed that when I fly IFR, ATC frequently shows me at an altitude that is 100' lower than what I see on my altimeter.  Figured that was close enough.  However, while going through the performance section of my '78 J model POH I actually took the time to look at the Altimeter Correction graph.  Since I usually find myself cruising around at 120 - 140 KIAS, turns out that I'm really 40 - 60' lower than my altimeter says.  That 'splains a lot.  Think I'll take an average of 50'.  I think from now on, if I'm supposed to fly at 5000', I'll fly at 5050' instead and see if it makes ATC happier.

Similarly, with gear and flaps down and at final approach speed, turns out I'm actually about 10 - 20' lower than the altimeter says.  If I'm IMC, I think I'll use a 20' correction and use minimums 20' higher than published.

How many of  you do that?

[Hank]

Never heard of correcting the altimeter based on airspeed, only for if the altimeter is reading incorrectly. Isn't the IFR requirement to be "field elevation ± 75 feet" anyway? It never hurts to know which way yours reads before departure.

[N201MKTurbo]

When you get your altimeter certified, you should get a paper with the errors at different altitudes. As far as airspeed is concerned, that corrects for the imperfect placement of the static ports. Burnulli works on the side of the fuselage too.

[ArtVandelay]

When you get your altimeter certified, you should get a paper with the errors at different altitudes. As far as airspeed is concerned, that corrects for the imperfect placement of the static ports. Burnulli works on the side of the fuselage too.

What am I suppose to do with that paper? POH or logbook?

[MB65E]

If you recently purchased your altimeter, it may have come with this chart and a 8130. I would make sure both items stay with the Airframe logs. I have a 3-ring binder for my logs with folders inside that I stuff other paperwork, overhaul reports, 8130's, and other useful items. Many times the logs won't reflect important items but the 8130's or shop notes will. As far as the logs themselves go, they fit nicely in a large 3-ringed pen case with zipper.

I would make sure you have had a recent IFR Pitot static check. Then have their notes from the repot logged in the AF log. 

75 feet is an FAR that needs to be upheld. 

-Matt

[carusoam]

I look at it simply...  Pitot static system gets tested every couple of years. They get adjusted to IFR standards by a qualified tech.

Set the kohlsmen's window according to what ATC says.  Fly the Altiude that ATC gives.

where is your error being measured from?  Are you using GPS altitude thinking that is a better idea?

GPS altitude would be good, if everyone changed to that standard.  That costs money and won't make you very popular.

Best regards,

-a-

[Bob - S50]

From what I'm reading, few if any people make the correction.

I'm not talking about errors in the altimeter.  I'm talking about inaccurate readings even with a perfect altimeter because of less than perfect static pressure measurement due to static port placement.  Check your POH and see if it has the Altimeter Correction graph that mine has ('78 J).  When I get home I'll try to post a picture of it.

When I look at the graph, it stops at 60 knots.  At that point, at SL, it shows a 14' correction and heading toward what I assume is 0' at 0 KIAS.  As speed increases it apparently creates an ever so slight reduction in air pressure over the static port.  This gives a slightly higher than actual altitude reading.  Since a given pressure differential means a larger error as altitude increases, the graph shows the need for a larger correction.  As speed increases, it apparently causes an even lower measured pressure.  At final approach speed of 80 KIAS the correction is 24' at SL and 35' at 10,000'.  At 120 KIAS the corrections for those altitudes are 41' and 57' respectively.  They max out at about 160 KIAS with errors of 49' and 66'.

[ArtVandelay]

Here is what Bob was referring to

[Andy95W]

1 hour ago, Bob - S50 said:

From what I'm reading, few if any people make the correction.

I'm not talking about errors in the altimeter.  I'm talking about inaccurate readings even with a perfect altimeter because of less than perfect static pressure measurement due to static port placement.  Check your POH and see if it has the Altimeter Correction graph that mine has ('78 J).  When I get home I'll try to post a picture of it.

When I look at the graph, it stops at 60 knots.  At that point, at SL, it shows a 14' correction and heading toward what I assume is 0' at 0 KIAS.  As speed increases it apparently creates an ever so slight reduction in air pressure over the static port.  This gives a slightly higher than actual altitude reading.  Since a given pressure differential means a larger error as altitude increases, the graph shows the need for a larger correction.  As speed increases, it apparently causes an even lower measured pressure.  At final approach speed of 80 KIAS the correction is 24' at SL and 35' at 10,000'.  At 120 KIAS the corrections for those altitudes are 41' and 57' respectively.  They max out at about 160 KIAS with errors of 49' and 66'.

Bob, it is far more likely that your Mode C altitude encoder is a tiny bit out of adjustment.  At your next transponder check, mention it to the avionics tech.

[carusoam]

Interesting point, Bob.

I outline below... the error is small, perfect precision at altitude can be hazardous, and precision when it counts doesn't use the altimeter at all...And I am not an expert, or a CFI.

1) there are a couple of places it becomes more important than other places.  Separation of planes and separation of the plane from the ground...

For separation of planes, we target 500' between IFR and VFR planes... The small published error plus the plane's unique differences is pretty small(?)

For separation from the ground, Precision approaches such as ILS and WAAS, don't use the altimeter when the precision is critical.

2) there are times when pilots use an intentional offset.  Traveling on a VFR airway with precision may not be the best idea.  You may encounter other precise pilots that don't look out the window.  George has amazing skills of being right on target.

3) I don't recall using offsets during IFR training.

4) Planes with only one static port and a small rigging problem would be more susceptible to static errors.  

5) Using the alternate static port and it's altitude error is also something that does get covered in IR training.

I'm only a PP, not a CFI.

Best regards,

-a-

[peevee]

center radar won't display a deviation from assigned altitude until 300 feet off....

[MB65E]

I'm not mentioning my "offset" lol.

-Matt

[Mooneymite]

A "purist" would make those adjustments detailed in the POH.

Other's of us are just glad to be close to the assigned altitude, corrected, or not...

Altimeter corrections really get to be serious business when it is very cold.  Your minimums need to be adjusted, or you could have a "tree event" on your approach.

https://www.nbaa.org/ops/airspace/20141211-notice-to-airmen-mandates-cold-weather-altitude-correction-reporting-at-272-airports.php

[peevee]

2 minutes ago, Mooneymite said:

A "purist" would make those adjustments detailed in the POH.

Other's of us are just glad to be close to the assigned altitude, corrected, or not...

Altimeter corrections really get to be serious business when it is very cold.  Your minimums need to be adjusted, or you could have a "tree event" on your approach.

https://www.nbaa.org/ops/airspace/20141211-notice-to-airmen-mandates-cold-weather-altitude-correction-reporting-at-272-airports.php

I assume you've seen the newish snowflake on approach plates for cold restricted airports?

[Mooneymite]

35 minutes ago, peevee said:

I assume you've seen the newish snowflake on approach plates for cold restricted airports?

I have.  However, I need a small hammer that pops out of the approach plate to hit me on the head for emphasis.  

[Glenn]

2 hours ago, N1395W said:

Bob, it is far more likely that your Mode C altitude encoder is a tiny bit out of adjustment.  At your next transponder check, mention it to the avionics tech.

This is likely the case.  You are reacting to the fact that the reported ATC observed altitude is different from what you observed in the cockpit, and concluded that it is a static system induced error.  Please consider that ATC is deriving their altitude information from the Mode C report from your transponder and encoding altimeter. Your encoding altimeter is connected to the same static system as the altimeter on the panel, and therefore should have the same error, if any. 

 

It is more likely that the altimeter setting (barometric pressure) at your airplane's location was slightly different from that which was being used by ATC's computers.

[peevee]

19 minutes ago, Mooneymite said:

I have.  However, I need a small hammer that pops out of the approach plate to hit me on the head for emphasis.  

Honestly for the ones close to me they're airports I'm not flying into in imc at any time, let alone when it's cold and icing is a concern so they affect me exactly not at all 

[Mooneymite]

13 minutes ago, peevee said:

..... so they affect me exactly not at all 

Yeah....I'm definitely not flying my Mooney in those temps voluntarily.  Unfortunately I have a very narrow minded boss at work that insists the temperature is "no factor" when he wants me to go somewhere in his plane.  

[Guest Mike261]

Transponders transmit your pressure altitude. (the altitude displayed on your altimeter when you dial 29.92 into the kollsman window)

the ATC computer applies correction before it displays on the controllers screen. sometimes the correction is different from the actual conditions, but generally they are pretty close. if your altimiter is reading correctly there shouldnt be too much of a discrepancy.

[Andy95W]

52 minutes ago, Mike261 said:

Transponders transmit your pressure altitude. (the altitude displayed on your altimeter when you dial 29.92 into the kollsman window)

the ATC computer applies correction before it displays on the controllers screen. sometimes the correction is different from the actual conditions, but generally they are pretty close. if your altimiter is reading correctly there shouldnt be too much of a discrepancy.

Yes, but to get it to transmit the correct pressure altitude, the encoder needs to be calibrated.  Most older/traditional encoders have 2 potentiometers (usually one high and one low).  More modern ones are done from a set up page.

But I agree that an error in the altimeter could also be an issue.  Luckily, this also gets checked and adjusted if necessary during the 24 month altimeter/ transponder check.

[Bob - S50]

Good discussion.  We just had our IFR certification done in August and the encoder was almost perfect at all altitudes.  I don't think it was off by more than 20' at any altitude.  I'm not 'worried' just a what some would call 'anal'.  If there is a known error, I tend to want to correct for it.  When I'm off altitude by 50' I tend to try and correct it.  As an old instructor used to say to me, "If you can maintain 5075' you can probably maintain 5000' too."

I had noticed over a period of time that ATC was routinely showing me 100' lower than my altimeter.  I've had a couple times that they asked me to verify my altitude. Usually when I was assigned an altitude at the floor of the Class B or crossing over the top of KSEA.  They tend to get nervous when they think they are vectoring you around below the Class B.  I just figured I'd try flying 50' high to correct for an approximation of the known error (I'm not going to read the chart every time I change altitude or airspeed) and see if it helps.

Even on a precision approach, you still fly to a minimum altitude.  Sometimes as low as 200' AGL.  At approach speed I'm only talking about a 20' error.  But you add up to a 75' altimeter error (which hopefully you noticed before takeoff by comparing your reading with field or touchdown elevation) and an error for cold temperature and you could be less than 100' AGL when you hit minumums and think you are at 200' AGL

TeeJayEvans: thanks for doing that, it's exactly what I was referring to.

Just food for thought.